A natural climate cycle
Over the last 800,000 years Earth’s climate has been cooler than today on average, with a natural cycle between ice ages and warmer interglacial periods. The transitions out of ice ages leading to an eventual global temperature change of around 4-7°C, took about 5,000 years.
Atmospheric CO2 levels have been very tightly linked to temperature throughout this cycle. In fact, the size of the global temperature changes can only be explained by including the varying greenhouse effect from CO2, without which temperature change would be have been much smaller.
Over the last 10,000 years (since the end of the last ice age) we have lived in a relatively warm period with stable CO2 concentration. This is the period during which human civilisation has flourished. Some regional changes have occurred – long-term droughts have taken place in Africa and North America, and the Asian monsoon has changed frequency and intensity – but these have not been part of a coherent global change.
The rate of CO2 accumulation due to our emissions in the last 200 years looks very unusual in this context (see chart below). Atmospheric concentrations are now well outside the 800,000 year natural cycle and temperatures would be expected to rise as a result.